The present invention relates to tubes of zirconium-based alloy for constituting all or the outer portion of a sheath for a nuclear fuel rod, or a guide tube for receiving a control rod in a fuel assembly. A major, although non-exclusive, application of the invention lies in the field of manufacturing sheathing tubes for fuel rods intended for pressurized water reactors or PWRs.
Until now, use has been made in particular of sheaths made of a zirconium-based alloy known as "Zircaloy 4" and that contains, by weight:
1.20% to 1.70% tin PA1 0.18% to 0.24% iron PA1 0.07% to 0.13% chromium PA1 transforming the hot ingot (e.g. by forging or by rolling) to obtain a bar; PA1 quenching the bar in water after heating it in an electric furnace or an induction furnace to between 1000.degree. C. and 1200.degree. C.; PA1 extruding a hollow billet blank after heating to a temperature lying in the range 600.degree. C. to 800.degree. C.; PA1 optionally applying heat treatment to the blank, in the range 560.degree. C. to 620.degree. C.; and PA1 cold rolling at least four times in the form of tubes of decreasing thickness with intermediate heat treatments and with final heat treatment at a temperature lying in the range 560.degree. C. to 620.degree. C., all heat treatments being performed under an inert atmosphere or under a vacuum.
where the total iron plus chromium content lies in the range 0.28% to 0.37%. Conventionally, the ratio of iron content to chromium content lies in the range about 1.38 to 3.42.
Usually, the oxygen content of a "Zircaloy 4" does not exceed 0.16%, and in general it is much smaller.
The mechanical strength of Zircaloy 4 sheaths has been found to be satisfactory, however their corrosion by pressurized water at high temperature has limited the length of time they can be kept in a reactor.
Sheaths have already been proposed made of a Zr-Nb alloy having about 2.5% niobium (U.S. Pat. No.4,717,534) which have been found to have good corrosion resistance in a high temperature aqueous medium. Unfortunately, that alloy has poor hot creep behavior. It has been improved by doping the alloy with an oxygen content lying in the range 0.10% to 0.16% by weight and by subjecting the sheath to final heat treatment for recrystallization. Nevertheless, the hot creep behavior of such an alloy remains less good than that of other sheathing materials.